Black-hole spacetimes are characterized by the presence of
null circular geodesics on which massless particles (photons, gravitons) can orbit
the central black hole. These closed light-like orbits play important physical
roles in astrophysical [1]}, [2]}, cosmological [3]}, and
theoretical [4]}, [5]}, [6]}, [7]}, [8]}, [9]}, [10]}, [11]}, [12]}
studies
of curved black-hole spacetimes.
